The present invention relates to a wire-dot print head of the spring-charge type used in a serial printer, and more particularly to a spring-charge type wire-dot print head having a configuration in which two or more electromagnets provided for respective print wires are energized through a common current conduction control element.
Various types of wire-dot print heads for use in a serial printer are known. One of them is a wire-dot print head of the spring-charge type in which a plate spring is resiliently deformed by attraction of an armature fixed to the plate spring due to a magnetic flux from a permanent magnet, and then released by energization of an electromagnet producing a magnetic flux cancelling the magnetic flux from the permanent magnet, so that the armature and a print wire fixed to the armature project to strike a printing paper through an ink ribbon. This causes transfer of ink from the ink ribbon onto the printing paper, effecting printing of one dot. In such a wire-dot print head, it is common that print elements, each comprising a print wire, an armature, a plate spring, an electromagnet, and a permanent magnet, are arranged in a ring. A problem associated with a prior-art wire-dot print head of this type is a magnetic interference and current induction between adjacent print elements, with attendant current induction and hence waste of power. This will be described in further detail with reference to the drawings.
FIG. 5 is a side view, half in section, showing the mechanical structure of a general spring-charge type wire-dot print head.
As illustrated, stacked on the peripheral surface of a disk-shaped rear yoke or base yoke 7 are an annular permanent magnet 4, an annular intermediate yoke 5, and an annular front yoke 6 having a first annular part 6b. A plate spring unit 3 comprises an annular support part 3b and radial parts 3a extending from the annular part 3b radially inward, i.e., toward the central axis CA of the disk-shaped rear yoke 7. Each of the radial parts 3a is also called a "plate spring". Fixed to the free end of each plate spring radial part 3a is an armature 2. The annular part 3b of the plate spring 3 is rigidly clamped between the annular part 6b of the front yoke 6 and the intermediate yoke 5. The front yoke 6 also has a second annular part 6c continuous with the first annular part 6b and extending from the first annular part 6b to be positioned in front of the armatures 2, and radial parts 6a extending from the second annular part 6c rearwardly (downwardly as seen in FIG. 5) to be positioned between adjacent armatures 2.
The armature 2 has a free end to which a rear end (base part) of a print wire 1 is rigidly attached. The tip (front end) of print wire 1 is arranged so that it can project through a guide aperture 11a of a wire guide 11 forming in the front end of the center of a front cover 15. The print wires 1 of the respective print elements are collected in the guide apertures 11a so that they are in a predetermined arrangement. The front cover 15 has an annular part 15b stacked on and fixed to the annular part 6b of the front yoke 6.
Located in the central portion of the rear yoke 7 are cores 8 on which coils 9 are wound, to form electromagnets. The cores 8 confront the rear surfaces of the armatures 1.
Although there are a plurality of the wires 1, the armatures 2 respectively supporting the wires 1, the plate spring radial parts 3a respectively supporting the armatures 2, and the cores 8 respectively associated with the armatures 2, FIG. 5 depicts only one of each for simplicity of illustration.
The print wires 1, the armatures 2, the plate springs 3, the permanent magnet 4, the intermediate yoke 5, the front yoke 6, the rear yoke 7, the cores 8 and the coils 9 form print elements. In the print head, the permanent magnet 4, the intermediate yoke 5, the front yoke 6, and the rear yoke 7 are common constituent parts, while the movable parts consisting of the print wires 1, the armatures 2 and the plate springs 3, and the electromagnets consisting of the cores 8 and the coils 9 are arranged in a ring on the rear yoke 7, to form a plurality of print elements.
FIG. 6 is a section along line A--A of FIG. 5. In the figure, reference marks 2a to 2c denote armatures, reference marks 8a to 8c are cores, and reference marks 9a to 9c denote coils. The armatures 9a and the core 8a; the armature 9b and the core 8b; and the armature 9c and the core 8c respectively form electromagnets.
FIG. 6 shows three electromagnets physically adjacent to each other in FIG. 5 and the corresponding armatures 2.
The printing operation of each printing element in the print head is as follows:
First, when the coil 9 in the above-described structure is not energized, the magnetic flux from the permanent magnet 4 passes through a magnetic path consisting of the intermediate yoke 5, the front yoke 6, the armature 2, the core 8 and the rear yoke 7, along a loop indicated by arrow P1. The armature 2 is attracted to the core 8 because the distance between the armature 2 and the core 8 is shorter than the distance between the armature 2 and the rearwardly (downwardly, as seen in FIG. 5 and FIG. 6) facing surface of the second annular part 6c of the front yoke 6, and because most of the magnetic flux between the armature 2 and the front yoke 6 passes through the gap between the armature 2 and the laterally facing surfaces of the radial parts 6a of the front yoke 6, rather than the rearwardly (downwardly, as seen in FIG. 5 and FIG. 6) facing surface of the second annular part 6c of the front yoke 6. As a result, the plate spring 3 is resiliently deformed or bent, and a strain energy is stored in the plate spring 3.
If, in this state, the coil 9 is energized, the magnetic flux developed in the core 8 by the coil 9 will cancel the magnetic flux developed by the permanent magnet 4. Therefore, the armature 2 will be released from the core 8. As a result, the plate spring 3a will restore its natural state, and the armature 2 and the print wire 1 are driven forward, and the tip of the print wire 1 will be ejected in the forward (upward as seen in the figure) direction through the guide aperture 11a in the front cover 15 and will print a dot forming part of a character or other print output onto a printing paper PP through an ink ribbon IR placed between the tip of the wire 1 and the printing paper PP on a platen PL.
The above describes the operation of one print element. By using a control circuit, not shown, to selectively energize the coils 9 of the respective print elements responsive to the print data, characters, numerals, etc. of dot configuration can be printed on the printing paper PP.
In this type of wire-dot print head in the prior art, two or more physically adjacent electromagnets are made to form a block, all the electromagnets are thereby divided into a plurality of blocks, and a common current conduction control element is provided for each block to control the energization of the coils of the electromagnets.
FIG. 7 is a diagram of a drive circuit for the coils 9 in the prior-art wire-dot print head. Three electromagnets, shown in FIG. 6, are made to form a block for control of energization.
As illustrated, first ends of the coils 9a to 9c are connected to the collector of a PNP transistor T-d for controlling the energization of the coils 9a to 9c. The emitter of the transistor T-d is connected to a first, or positive terminal of a power supply E supplying electric energy to the coils 9a to 9c. Second ends of the coils 9a to 9c are connected to collectors of NPN transistors T-a, T-b and T-c which control the energization of the coils 9a to 9c individually and the anodes of diodes D-a, D-b and D-c.
The cathodes of the diodes D-a, D-b and D-c are connected to the first terminal of the power supply E. The emitters of the transistors T-a, T-b and T-c are connected to the first ends of the coils 9a to 9c through a diode D-d for conducting a circulating current. The emitters of the transistors T-a, T-b and T-c are also connected to the ground G. The second, or negative terminal of the power supply is also grounded.
Although not illustrated, the circuit configuration in other blocks is similar.
The operation of the above-described configuration will now be described.
As an example, it is assumed that the coil 9a is energized so that the armature 2a associated with the coil 9a and the print wire 1 are projected to effect printing.
The printing of one dot, in one printing cycle can be divided into three stages. The first stage lasts from the commencement of excitation of the selected electromagnet and until about the commencement of forward movement of the associated armature and the print wire. The second stage lasts from about the commencement of the forward movement of the armature and the print wire and until about the impact of the print wire on the printing paper. The third stage lasts from about the impact of the print wire on the printing paper and until the current due to an electromotive force induced in the coil ceases. The commencement of the forward movement of the armature and the print wire, and the impact of the print wire on the printing paper can be detected by means not shown, or assumed to occur at predetermined timings, by use of timing elements.
FIG. 8 is a diagram showing the waveforms of the currents flowing through the coil 9a in the first, second and third stages. The parts [1], [2] and [3] correspond to the first, second and third stages, respectively.
In the first stage, a signal DT1 applied to the transistor T-d is Low (Active), so the transistor T-d is ON, and also a signal DT-2 applied to the transistor T-a is High so the transistor T-a is ON. The other transistors T-b and T-c are kept OFF. The current flows, as shown by arrow [1], i.e., from the first terminal of the power supply E, then through the transistor T-d, then the coil 9a, and then the transistor T-a, and then to the ground. Because of this current, the electromagnet is excited to generate a magnetic flux and the associated armature and the print wire begin to move.
In the second stage, the signal DT1 is High (Inactive), while the signal DT2 is High, so the transistor T-a is kept ON, while the transistor T-d is OFF. Although the coil 9a is isolated from the power supply E, an electromotive force induced in the coil 9a causes a current to flow through the path shown by arrow [2], i.e., from the coil 9a, then through the transistor T-a, and then the diode D-d, and then back to the coil 9a.
In the third stage, the signal DT2 applied to the transistor T-a also Low, so the transistor T-a is also OFF. Because of the electromotive force still induced in the coil 9a, a current flows through the path as shown by arrow [3], from the ground, then through the diode D-a, then the coil 9a, and then the diode D-a, and then to the first terminal of the power supply E. This current rapidly diminishes.
A problem associated with the prior art described above is that, induced by the electromagnet having a coil being energized, a current also flows through a coil of an electromagnet which is physically adjacent to the electromagnet having the coil being energized.
This phenomenon will be described in further detail.
That is, the canceling magnetic flux that is created when the coil 9a of the electromagnet is energized, not only flows through the core 8a in a direction opposite to the attracting magnetic flux of the permanent magnet 4, but also flows through the adjacent armatures 2b and 2c and cores 8b and 8c, along loops P2 and P3, to cause a magnetic interference.
As a result, a current flows through the coils 9b and 9c of the adjacent electromagnets, and whereas only the coil 9a in FIG. 7 is energized induction currents also flow through the coils 9b and 9c. That is the currents due to the magnetic interference flow through the transistor T-d, then through the coils 9b and 9c, then through the diodes D-b and D-c.
Because of the induction current which flows through the coils of the adjacent electromagnet that is not energized, the power is wasted.